A tide dial, also known as a Mass^[2] or scratch dial,^[3][4] is a sundial marked with the canonical hours rather than or in addition to the standard hours of daylight. Such sundials were particularly common between the 7th and 14th centuries in Europe, at which point they began to be replaced by mechanical clocks. There are more than 3,000 surviving tide dials in England and at least 1,500 in France.

The name tide dial preserves the Old English term tīd, used for hours and canonical hours prior to the Norman Conquest of England, after which the Norman French hour gradually replaced it. The actual Old English name for sundials was dægmæl or "day-marker".

Jews long recited prayers at fixed times of day. Psalm 119 in particular mentions praising God seven times a day,^[5][6] and the apostles Peter and John are mentioned attending afternoon prayers.^[7] Christian communities initially followed numerous local traditions with regard to prayer, but Charlemagne compelled his subjects to follow the Roman liturgy, and his son Louis the Pious imposed the Rule of St Benedict upon their religious communities.

The canonical hours adopted by Benedict and imposed by the Frankish kings were the office of matins in the wee hours of the night,^{[lower-alpha 1]} Lauds at dawn, Prime at the 1st hour of sunlight, Terce at the 3rd, Sext at the 6th, Nones at the 9th,^[6] Vespers at sunset,^[10] and Compline before retiring in complete silence.^[11] Monks were called to these hours by their abbot^[12] or by the ringing of the church bell, with the time between services organised in reading the Bible or other religious texts, in manual labour, or in sleep.

The need for these monastic communities and others to organize their times of prayer prompted the establishment of tide dials built into the walls of churches. They began to be used in England in the late 7th century and spread from there across continental Europe through copies of Bede's works and by the Saxon and Hiberno-Scottish missions. Within England, tide dials fell out of favour after the Norman Conquest.^[13] By the 13th century, some tide dials – like that at Strasbourg Cathedral – were constructed as independent statues rather than built into the walls of the churches. From the 14th century onwards, the cathedrals and other large churches began to use mechanical clocks and the canonical sundials lost their utility, except in small rural churches, where they remained in use until the 16th century.

There are more than 3,000 surviving tide dials in England^{[14][lower-alpha 2]} and at least 1,500 in France,^[16] mainly in Normandy, Touraine, Charente, and at monasteries along the pilgrimage routes to Santiago de Compostela in northwestern Spain.

With Christendom confined to the Northern Hemisphere, the tide dials were often carved vertically onto the south side of the church chancel at eye level near the priest's door. In an abbey or large monastery, dials were carefully carved into the stonen walls, while in rural churches they were very often just scratched onto the wall.

Some tide dials have a stonen gnomon, but many have a circular hole which is used to hold a more easily replaced or adjusted wooden gnomon. These gnomons were perpendicular to the wall and cast a shadow upon the dial, a semicircle divided into a number of equal sectors. Most dials have supplementary lines marking the other 8 daytime hours, but are characterized by their noting the canonical hours particularly. The lines for the canonical hours may be longer or marked with a dot or cross. The divisions are seldom numbered.

Dials often have holes along the circumference of their semicircle. As additional gnomons were needless and these holes are often quite shallow, T.W. Cole suggests they were used as markers to quickly and easily reconstruct the tide dials following a fresh whitewash of the church walls with chalk or lime.^[17]

Proper tide dials prominently displaying the canonical hours:

• A photograph of St Andrew's tide dial in Bishopstone, East Sussex, England
• Tide dial at St Peter's in Bywell, Northumberland, England
• Tide dial at St Andrew's at Hempstead near Lessingham, Norfolk, England
• Tide dial of Notre Dame in Uzeste, France
• Tide dial of the church in Dahmen, Germany
• Tide dial of St Peter's in St-Pierre-du-Palais, France
• The medieval tide dial at the church in Pattensen, Germany, with a modern gnomon inserted
• The tide dial at St Marien's in Homberg, Germany
• The tide dial at St Beuno's in Clynnog Fawr, Wales (10–11th c.)
• Tide dial of the Saxon Church in Corhampton, Hampshire, England^[23] (11th c.)
• A tide dial on the church at Berka, Germany (15th c.)

Other ecclesiastical sundials ("Mass dials") used to determine times for prayer and Mass during the same period:

• The Bangor Sundial in Bangor, Northern Ireland (c. 900)
• The mosque sundial at the former Doliskana monastery in Turkey's Artvin Province (10th c.)
• Sundial with a triangular gnomon at St George's in Damerham, Hampshire, England
• Two sundials on a buttress at St Mary's in Chalgrove, Oxfordshire, England
• Sundial with half-hour marks at St Martin's in Cheselbourne, Dorset, England
• St Plechelm's sundial in Oldenzaal in the Netherlands (12th c.)
• The church sundial in Arbo, Spain
• Sundial preserved amid a newer wall at SS Peter & Paul's in West Clandon, Surrey, England
• San Lorenzo's "Knifegrinder" sundial in Genoa, Italy^[24]
• The sundial at SS Peter & Martin's in Condé-on-the-Ifs, France
• The sundial on the monastery church at Bonaval, Spain
• A sundial at St Marien's in Stelzen, Germany, marked with Runic numerals
• A sundial at the Istanbul Museum marked with Greek numerals
• The sundial at Zvartnots Cathedral, marked with Armenian numerals
• The sundial on the church patio at San Jerónimo Tlacochahuaya, Mexico (16th c.)